1. Field of the Invention
This invention relates to a microporous membrane composed of an ultra-high-molecular-weight polyolefin, more particularly to a microporous polyolefin membrane of high permeability and novel structure, and a method of producing the same.
2. Description of the Prior Art
Microporous membranes are widely used in various applications such as battery separators (in particular, lithium ion type primary and secondary batteries), large-sized battery separators or electrolytic capacitor separators for electric cars or the like, various types of separation membranes (e.g., for water treatment, ultrafiltration, microfiltration and reverse osmosis), various types of filters, moisture-permeable and waterproof clothes and the base materials thereof, etc.
Microporous polyolefin membranes are produced, e.g., by an extraction process comprising the steps of mixing a polyolefin with an organic solvent and inorganic powder such as finely powdered silica, melting and molding the mixture, and extracting the organic solvent and inorganic powder. This method needs a process of extracting the inorganic powder, and permeability of the membrane produced depends largely on particle size of the inorganic powder used and is difficult to control.
Recently, various processes have been proposed to produce high-strength, microporous membranes which contain an ultra-high-molecular-weight polyolefin. For example, Japanese Patent Laid-Open Nos. 60-242035, 61-195132, 61-195133, 63-39602, 63-273651, 3-64334, and 3-105851 disclose processes to produce microporous membranes by forming a gel-like sheet from a heated solution of a polyolefin composition containing an ultra-high-molecular-weight polyolefin dissolved in a solvent, stretching it while heating, and removing the solvent by extraction. These processes give the microporous polyolefin membranes characterized by a narrow pore size distribution and small pore size, which are used for battery separators or the like.
Recently, lithium ion type secondary batteries are required to have a property of high energy density, and microporous membranes of polyolefin having a shut-down function are widely used for the battery separators, to meet the increasing demands for the batteries. However, for the special battery purposes, e.g., those requiring high output at low temperature, a microporous polyolefin membrane of small pore size may cause problems, e.g., increased inner resistance of the battery. Therefore, the microporous polyolefin membranes high in safety and ion permeability are in demand.
It is an object of the present invention to provide a microporous polyolefin membrane of high permeability and novel structure. It is another object of the present invention to provide a method of producing the same.
The inventors of the present invention have found, after having extensively studied to solve the above problems, that a microporous polyolefin membrane of novel structure, having large pores, high permeability coming from the polyolefin""s lamella crystals oriented in the direction perpendicular to the plane parallel to the microporous membrane, and low thermal shrinkage, can be produced by extruding a solution of a specific content of ultra-high-molecular-weight polyolefin or the composition containing an ultra-high-molecular-weight polyolefin dissolved in a solvent, into a gel-like formed article, thermally treating the article under a stress in such a way to prevent shrinkage occurring in a specific temperature range at least in one direction on the membrane, and removing the solvent, to reach the present invention.
The present invention provides a microporous polyolefin membrane with the fine fibrils, composed of (A) a polyolefin having a weight-average molecular weight of 5xc3x97105 or more or (B) a composition containing this polyolefin, connected to each other, wherein its average pore size is 0.05 to 5 xcexcm, and the crystal lamellas of the polyolefin inclined at an angle xcex8 of 80 to 100xc2x0 to the plane constituting the membrane account for at least 40% of the total lamellas both on the section cut in the mechanical direction and on the section cut in the direction perpendicular to the mechanical direction and in the thickness direction. The present invention also provides a method of producing the same microporous polyolefin membrane, comprising the steps of extruding the solution, composed of 10 to 50 weight % of (A) a polyolefin having a weight-average molecular weight of 5xc3x97105 or more or (B) a composition containing this polyolefin and 50 to 90 weight % of a solvent, into a gel-like formed article; thermally setting the article, with or without stretching, at least at the crystal dispersion temperature of the (A) polyolefin having a weight-average molecular weight of 5xc3x97105 or more or (B) composition containing this polyolefin, but at melting point of the (A) polyolefin having a weight-average molecular weight of 5xc3x97105 or more or (B) composition containing this polyolefin plus 30xc2x0 C. or lower; and removing the solvent.
The crystal dispersion temperature is determined by measuring the temperature characteristics of the dynamic viscoelasticity, in accordance with ASTM D4065, and melting point is the peak temperature observed by a DSC, in accordance with JIS K7211 (the same in the following pages).